This invention generally relates to a rotary tool connecting assembly for transmitting torque to a rotary tool while allowing axial linear movement to occur between the tool and the connecting assembly, and is specifically concerned with a tap driver assembly.
Tap driver assemblies are known in the prior art. Such prior art tap drivers generally comprise a chuck body having one end that is detachably connectable to a torque-generating spindle, and which includes on its other end an adapter receiver assembly which is capable of coupling on to the shank end of a threading tap through an adapter. In order for the tap to properly cut threads in a metal work piece without undercutting or overcutting the threads, it is essential that the tap driver assembly include some sort of mechanism for allowing the adapter receiver which holds the shank end of tap to move along the axis of rotation of the chuck body when the tap performs a thread cutting operation. To this end, the adapter receivers of prior art tap driver assemblies are slidably movable within a complementarily-shaped recess which is generally cylindrical in shape, and concentrically aligned with the axis of rotation of the chuck body. During the operation of such tap drivers, the feed rate of the rotating spindle that applies torque to the chuck body is set slightly lower than the screw pitch of the tap so that the resulting threads created by the tap will not be distorted by overcutting, while the ability of the tap to move axially away from the chuck body during the cutting operating insures that the threads will not be distorted by undercutting.
While many prior art tap driver assemblies are capable of achieving their intended purpose, both the operating abilities and useful life span of such assemblies have been limited by the mechanisms in such assemblies which transmit torque between the tap adapter receiver and the chuck body while at the same time affording the necessary degree of axial free movement between these two components. One of the simplest of these prior art mechanisms employs a keyway that fits within a complementarily-shaped slot between the adapter receiver and the recess in the chuck body which it is slidably received within. While such a mechanism is simple and inexpensive to manufacture, the metal-to-metal contact between the keyway and the slot which receives it substantially limits the amount of torque that this mechanism can transmit before the localized pressure applied by the keyway to the slot frets the slot surfaces, causing binding and even seizing to occur at relatively modest levels of torque. To solve this problem, some prior art tap driver assemblies have replaced the keyway and slot combination with a plurality of ball bearings which are captured within one or more pairs of axially-aligned grooves disposed between the outer surface of the adapter receiver and the inner surface of the recess in the chuck body which receives it. While the use of ball bearings provides a mechanism which is capable of transmitting larger amounts of torque while still providing axial movement between the tap adapter receiver and the recess within the chuck body which houses it, the applicants have observed that the one-point contact between the ball bearings and their respective grooves can still cause fretting to occur in the walls of the grooves, which in turn causes binding to occur at various points along the axial path between the tap and the chuck body during a tapping operation. If this binding becomes severe enough, undercutting of the threads created by the tap can occur at the binding points.
To overcome the shortcomings associated with the use of ball bearings in a torque-transmitting mechanism in a tap driver, other prior art designs have attempted to use roller bearings. In contrast to the point contact that ball bearings exert on the walls of the grooves that capture them, roller bearings are capable of applying a line contact all the way across their lengths which would appear to offer some potential solution to the fretting caused by point contact. Unfortunately, no prior art tap driver assembly has yet managed to exploit all of the potential that the use of roller bearings would appear to offer in a tap driver torque-transmitting mechanism. For example, while FIGS. 1 through 3 of the Hopkins U.S. Pat. No. 3,806,973 disclose a tap driver which utilizes a plurality of roller bearings which orbit about an oval groove cut between a driving member and a driven member which in turn holds the shank of a tap, there is substantial friction between the circular end surfaces of these roller bearings and the grooves which capture them, as well as significant friction between the bearings themselves. Moreover, the precision oval cut required in both the driving and driven members requires relatively difficult and time consuming machining operations which in turn substantially adds to the cost of manufacturing this tool. Finally, the fact that all the bearing surfaces are formed by vee-grooves cut in the major housing components of the tool necessarily limits the hardness of the bearings surfaces to the hardness of the tool steel used to form these housing components, which cannot be hardened above a certain Rockwell level before the resulting embrittlement would interfere with the normal operation of these components.
Clearly, there is a need for a tap driver assembly which is capable of transferring large amounts of torque between the chuck body and the tap- holding adapter receiver while still allowing for free axial movement between the adapter receiver and the chuck body. Ideally, such a mechanism would be capable of transmitting a maximum amount of torque while imposing only a minimum of friction in the axial direction of movement without causing fretting to occur between the bearings and the surfaces they engage during the operation of the assembly. It would be desirable for such a tap driver assembly to fully exploit the torque transmitting potential of roller bearings in a tool that is relatively simple and inexpensive to manufacture, but yet has a long durability. Finally, it would be desirable if the design of the ta driver assembly allowed the bearing surfaces to be hardened separately from the other components of the assembly which must maintain some degree of elasticity.